The margins beneath the aircraft shrink as the aircraft descends toward the runway, and the sequence of minimum obstacle clearances in the primary area is an exam favourite:
Arrival and initial segments — 300 m (984 ft) of clearance.
Intermediate segment — 150 m (492 ft).
Final approach segment of a non-precision approach with a FAF — 75 m (246 ft).
The funnel narrows deliberately: every segment trades margin for precision as the aircraft closes with the ground.
The initial approach segment has one job: manoeuvring the aircraft into position to enter the intermediate segment, losing altitude, closing with the procedure's axis, absorbing whatever direction the aircraft arrived from. Because its geometry can be generous, the designer bounds the join to the next segment: at the intermediate fix, the maximum interception angle between the initial and the intermediate segment is 90 degrees for a precision approach and 120 degrees for a non-precision approach, a tighter funnel where the following final segment demands the tightest alignment.
The intermediate segment is the breathing space: its main task is preparing the aircraft for the final approach, adjusting speed, configuration and position so that the final segment is begun stabilised. It is therefore designed flat or nearly so: the descent is shallow or level while flaps travel and the approach is armed, which is also why its obstacle clearance can halve to 150 m.
Final approach: two possible aims
The final approach segment is where alignment and descent for landing are completed. It can aim at one of exactly two things: a straight-in landing on the runway the procedure serves, or, where the final track cannot meet the straight-in alignment criteria, a circle-to-land: visual manoeuvring around the aerodrome to a landing runway, the next lesson's subject.
On an ILS the final approach begins not at a fix but at a point: the final approach point (FAP), where the aircraft, level at the intermediate approach altitude, intercepts the nominal glide path. And the ILS carries a built-in demotion clause the exam loves: if the glide path fails during the approach, the ILS becomes a non-precision approach — the pilot continues, if able, as a localizer-only procedure, and the localizer-only OCA/H and minima apply from that moment. The vertical guidance is gone, so the precision-approach numbers go with it.
The missed approach
Every instrument approach carries its escape route, and its main task is simple to state: to bring the aircraft from the missed approach point back to a safe altitude with obstacle clearance protected throughout, to a point where a new approach, a holding, or a return to en-route flight can begin. The missed approach point (MAPt) is defined as the point in the procedure at or before which the prescribed missed approach must be initiated so that the minimum obstacle clearance is not infringed. On a chart the MAPt is established either by a navigation facility, a fix, or a specified distance from the final approach fix (a distance flown by timing where no fix exists); on a precision approach it is the point where the decision altitude/height is reached on the glide path.
The procedure itself runs in three phases, each with its own geometry:
Initial phase — from the MAPt to the start of climb (SOC): the go-around is begun, the aircraft rotates from descent to climb, and no turns are permitted.
Intermediate phase — from the SOC to the first point where 50 m (164 ft) of obstacle clearance is obtained and can be maintained: the climb continues, the track may be changed by a maximum of 15 degrees, and 30 m (98 ft) of clearance is protected through the phase.
Final phase — from that point until a new approach, holding or en-route flight begins: 50 m of clearance is maintained, and turns may now be specified.